The present invention relates to a device for automatically measuring deep holes and deep grooves on a wafer during fabrication of a semiconductor device. First, the fact that deep holes and deep grooves are increased in a semiconductor-device fabrication process will be described, and then, automatic measurement devices for semiconductor devices will be described.
In recent years, the importance of dimension measurement of the bottoms of holes and grooves having high aspect ratios is increased in the semiconductor device fabrication process. For example, Non-Patent Literatures 1 and 2 describe methods in which a number of conductive films and insulating films are stacked on each other and are collectively subjected to etching, in order to reduce the cost of lithography for flash memories, and the aspect ratios of holes and grooves are 30 or larger in the process of fabrication of such configurations. Furthermore, Non-Patent Literature 3 describes a configuration in which a region between a source and a drain of a transistor, that is, an active region, is surrounded by a gate wire, and the aspect ratios of holes and grooves are also 30 or larger in the process of fabrication of this configuration.
In semiconductor-device mass-production factories, in order to manage the semiconductor-device fabrication process, the dimensions of tops and bottoms of holes and grooves are measured. This dimension measurement can be performed using a scanning electron microscope (SEM). In the SEM, electrons emitted from an electron source are accelerated, are converged by an objective lens to which an excitation current is applied, and are radiated onto the surface of a wafer. The stream of the electrons is called an electron beam. Through the irradiation of the electron beam, secondary electrons are generated from the wafer. When the electron beam is scanned on the wafer surface through electromagnetic deflection to obtain the secondary-electron signal strength, an electron microscope image (SEM image) reflecting the shape of the wafer is acquired. The dimensions of a pattern on the wafer are determined from this SEM image and the magnification of the image.
In a scanning electron beam device for automatically measuring the dimensions of a pattern on a wafer using an electron beam, the stage on which the wafer is loaded is horizontally moved, and the electron beam is radiated onto a predetermined measurement point, thereby measuring the dimensions of the pattern. Because of a warp of the wafer or a tilt of the stage, when the stage is horizontally moved, the height of the wafer varies. The variation in the height of the wafer causes out-of-focus, that is, image blurring, thus worsening the precision of pattern dimension measurement. Therefore, as disclosed in Patent Literature 1, the working distance (WD) or the strength of an electromagnetic lens or an electrostatic lens is changed to achieve the in-focus state. A description thereof will be given below in more detail. If focusing is performed based on a SEM image acquired by radiating the electron beam onto the wafer, image blurring is solved, but the amount of electrons to be radiated is increased, thereby causing problems of electrostatic charge, contamination, and resist shrinkage of the wafer. In order to avoid these problems caused by the electron-beam irradiation, the length of time of the electron-beam irradiation for focusing needs to be reduced; for this purpose, before the focusing based on a SEM image, it is necessary to measure the height of the wafer using a height sensor and to perform rough focus adjustment, in advance. The rough focus adjustment is performed through electromagnetic focusing that is performed by adjusting an excitation current for the objective lens. In order to adjust the excitation current according to the height of the wafer, it is desired that the reciprocal of the focal distance, that is, the lens strength, be always proportional to the excitation current.